IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) specification for implementing wireless local area network (WLAN) communication in the Wi-Fi (2.4, 3.6, 5, and 60 GHz) frequency bands. The 802.11 family consists of a series of half-duplex over-the-air modulation techniques that use the same basic protocol. The standards and amendments provide the basis for wireless network products using the Wi-Fi frequency bands. For example, IEEE 802.11ac is a wireless networking standard in the IEEE 802.11 family providing high-throughput WLANs on the 5 GHz band. Significant wider channel bandwidths (20 MHz, 40 MHz, 80 MHz, and 160 MHz) were proposed in the IEEE 802.11ac standard. The High Efficiency WLAN study group (HEW SG) is a study group within IEEE 802.11 working group that will consider the improvement of spectrum efficiency to enhance the system throughput in high-density scenarios of wireless devices. Because of HEW SG, TGax (an IEEE task group) was formed and tasked to work on IEEE 802.11ax standard that will become a successor to IEEE 802.11ac. Recently, WLAN has seen exponential growth across organizations in many industries.
Orthogonal Frequency Division Multiple Access (OFDMA) is introduced in HE WLAN to enhance the user experiences by assigning subsets of subcarriers to different users, allowing simultaneous data transmission by several users. In OFDMA, each user is assigned with a group of subcarriers called a resource unit (RU). In HE WLAN, a wireless station (STA) can transmit one minimum size RU (which is about 2 MHz bandwidth) in uplink and downlink OFDMA. Compared to its 20 MHz preamble, the power density of its data portion is 9 dB higher than its preamble. This narrow band uplink OFDMA signal is hard to be detected by CCA because CCA is operated on bandwidth that is greater or equal to 20 MHz. Therefore, one STA can experience 9 dB higher interferences on subcarriers in a particular narrow band than other subcarriers. It can be seen that narrow band interferences are intrinsic in HE WLAN. A scheme to deal with such narrow band interferences is needed.
In Multi-User (MU) transmissions, performance of HE-SIG-B is encoded using 1× symbol duration. As a result, its performance is worse than data symbol with 4× symbol duration when used the same modulation and coding scheme (MCS). A more robust modulation scheme is needed for HE-SIGB. In addition, to extend the range for outdoor scenarios, a new modulation scheme that can operate at lower SNR than MCS0 is also desired.
Dual Sub-Carrier Modulation (DCM) modulates the same information on a pair of subcarriers. DCM can introduce frequency diversity into OFDM systems by transmitting the same information on two subcarriers separated in frequency. DCM can be implemented with low complexity and provide better performance than existing modulation schemes used in WLAN. DCM enhances the reliability transmissions, especially under narrow band interferences. In information theory, a low-density parity-check (LDPC) code is a linear error correcting code, a method of transmission over a noisy transmission channel. LDPC codes are commonly used in WLANs and OFDM systems. In HE WLAN, when bandwidth is larger than 20 MHz, LDPC becomes the mandatory error control coding scheme.
In the next generation WLAN systems that is based on upcoming IEEE 802.11ax standards, each STA can transmit signals using one or more RU. When DCM is applied for a given RU, a transmission procedure with a new LDPC tone mapping scheme is desired to facilitate the enhanced transmission reliability under DCM.